The present invention generally relates to modulation systems used in radio communications, and more particularly to a modulation system which is applicable to a data transmission of a communication system using a frequency shift keying (FSK) modulation technique and carries out a modulation of a binary value or greater using the FSK modulation technique.
One of the problems associated with a transmission characteristic of the radio communication is a leakage power to an adjacent channel. This leakage power to the adjacent channel refers to the power which leaks to the adjacent channel during the data transmission. If the leakage power to the adjacent channel is outside a range of a standard value, undesirable effects are introduced by the leakage power which acts as an interference wave with respect to other channels. For this reason, the band must be limited so as to transmit only the desired frequencies.
In a transmitter which makes the data transmission, a high-frequency component is eliminated by use of a lowpass filter or the like, and a frequency deviation is changed slowly or smoothly, so as to satisfy the above described standard.
FIGS. 1A and 1B respectively are diagrams for explaining the slow frequency deviation. More particularly, FIG. 1A shows a frequency versus level characteristic, and FIG. 1B shows a time versus frequency characteristic. In FIG. 1A, the ordinate indicates the level and the abscissa indicates the frequency. On the other hand, in FIG. 1B, the ordinate indicates the frequency and the abscissa indicates the time.
In FIG. 1A, f0 indicates a modulated wave (carrier frequency), and f0+.DELTA.f and f0-.DELTA.f indicate adjacent channels. In the case of a binary FSK modulation, the modulation is carried out by making f1 correspond to "1" and f2 correspond to "0", for example. Accordingly, in the case of the adjacent channels shown in FIG. 1A, the channels on both sides are completely separated from the modulated wave.
FIG. 1B is a diagram showing an overwrite locus, and a frequency deviation from f1 to f2 and a frequency deviation from f2 to f2 are smooth. In the case of such a slow frequency deviation, the leakage power to the adjacent channel is uneasily generated.
On the other hand, in the case of a fast frequency deviation, the modulated wave f0 enters into the adjacent region as shown in FIG. 2A, and the leakage power to the adjacent channel is generated. FIG. 2B shows a time versus frequency characteristic for the case shown in FIG. 2A. In this case, the leakage power becomes an interference wave with respect to the adjacent channel and causes undesirable effects when the leakage power becomes outside the range of the standard value.
On the other hand, in a receiver which receives the data transmission, it is possible to carry out a more stable demodulation if the frequency deviation is faster (ideally a square wave). Therefore, there is a demand to satisfy the standard of the leakage power to the adjacent channel and at the same time satisfy a frequency deviation advantageous to the receiver.
There are various methods of limiting the band in the radio communication, as described in the following.
A base band filtering method smoothens the modulated data by use of a lowpass filter.
A radio frequency (RF) filtering method uses a bandpass filter to pass only the necessary transmitting frequencies.
A digital signal processor (DSP) filtering method calculates a signal passed through a lowpass filter from the demodulated data.
A DSP direct modulation method outputs a signal which is modulated into an intermediate frequency from the modulated data.
FIGS. 3A through 3C are diagrams for explaining conventional band limiting methods. FIG. 3A shows the method which combines the base band filtering method and the RF filtering method described above, FIG. 3B shows the method which combines the DSP filtering method and the RF filtering method, and FIG. 3C shows the method which combines the DSP direct modulation method and the RF filtering method.
In FIG. 3A, a pulse shaped input data is input to a lowpass filter (LPF) 1 and smoothened. The smoothened output signal of the LPF 1 is input to a frequency modulation (FM) circuit 2 and frequency modulated. On the other hand, a central processing unit (CPU) 3 supplies a predetermined data to a phase locked loop (PLL) circuit 4. An output of the PLL circuit 4 is input to a voltage controlled oscillator (VCO) 5. An output of the VCO is fed back to the PLL circuit 4. As a result, the VCO 5 outputs a frequency signal having a fixed phase.
An output of the FM circuit 2 and the output of the VCO 5 are input to a mixer 6 and mixed. Hence, a spectrum signal having levels separated for every frequency as shown below the mixer 6 in FIG. 3A is output from the mixer 6. This spectrum signal is input to a RF filter circuit 7. The RF filter circuit 7 passes only a predetermined frequency component, and a signal having a single peak at a specific frequency as shown on the right of the RF filter circuit 7 in FIG. 3A is output from the RF filter circuit 7. Therefore, the modulated pulse data is converted into a frequency data.
In FIG. 3B, when a modulated pulse data is input to a DSP 10, the DSP 10 carries out an operation similar to that of a LPF by a digital processing. An output of the DSP 10 is converted into an analog signal by a digital-to-analog (D/A) converter 11. An output of the D/A converter 11 is input to the FM circuit 2 which carries out a frequency modulation dependent upon the input signal.
On the other hand, the CPU 3 supplies a predetermined data to the PLL circuit 4. An output of the PLL circuit 4 is input to the VCO 5. An output of the VCO 5 is fed back to the PLL circuit 4. As a result, the VCO 5 outputs a frequency signal having a fixed phase.
An output of the FM circuit 2 and the output of the VCO 5 are input to the mixer 6 and mixed. Hence, a spectrum signal having levels separated for every frequency as shown above the mixer 6 in FIG. 3B is output from the mixer 6. This spectrum signal is input to the RF filter circuit 7. The RF filter circuit 7 passes only a predetermined frequency component, and a signal having a single peak at a specific frequency as shown on the right of the RF filter circuit 7 in FIG. 3B is output from the RF filter circuit 7. Therefore, the modulated pulse data is converted into a frequency data.
In FIG. 3C, a modulated pulse data is input to a direct modulation DSP 20. The DSP 20 carries out a digital processing to calculate and output a frequency spectrum signal directly from the input pulse data. The frequency spectrum signal from the DSP 20 is input to the RF filter circuit 7 which passes only a predetermined frequency component. A signal having a single peak at a specific frequency as shown on the right of the RF filter circuit 7 in FIG. 3C is output from the RF filter circuit 7. Therefore, the modulated pulse data is converted into a frequency data.
The baseband filtering method carries out an analog processing, and a change in the frequency deviation curve depends on the data string to the modulated and the characteristics of the elements used. When passing the single pulse and the pulses having a short period through the LPF out of the modulated data string, there is a problem in that the actual frequency deviation point will not be reached. As a result, the demodulation carried out at the receiver becomes unstable.
FIGS. 4A through 4C are diagrams for explaining the frequency deviation of the modulated data. FIG. 4A shows the modulated data having the short period, FIG. 4B shows the modulated data having a single change, and FIG. 4C shows the modulated data having a long period. In FIGS. 4A through 4C, L1 indicates a deviation point of f0+.DELTA.f, and L2 indicates a deviation point of f0-.DELTA.f. When subjecting the modulated data having the short period shown in FIG. 4A and the modulated data having the single change shown in FIG. 4B to the frequency deviation, the frequency deviation point will not be reached. In the case shown in FIG. 4A, both the deviation points L1 and L2 are not reached. In the case shown in FIG. 4B, the deviation point L2 is reached, but the deviation point L1 is not reached. On the other hand, in the case of the modulated data having the long period shown in FIG. 4C, both the deviation points L1 and L2 are reached.
Returning now to the description of the band limiting methods, the RF filtering method uses a bandpass filter which is essential to pass only the necessary transmitting frequencies. The DSP filtering method and the DSP direct modulation method have become more popular in the recent years. The DSP filtering method prestores the frequency deviation curve corresponding to the modulated data, and carries out the frequency deviation. The DSP direct modulation method calculates the frequency deviation curve corresponding to the modulated data in real-time, and carries out the frequency deviation.
However, in the case of the radio communication, the specification specifies that "the cutoff frequency is XX Hz or greater" or the like, and the frequency deviation is not carried out by taking into consideration and to suit the data string, the leakage power to the adjacent channel and the reception characteristic.
(1) A description will be given of a case where the LPF passes the single data string shown in FIG. 4A or the data string having a short period shown in FIG. 4B.
In this case, the data string can be regarded as being a square wave pulse from the point of view of a distortion wave A.C. waveform. The square wave pulse can be represented by a family of sets of the high-frequency component by a Fourier series, and thus, the high-frequency component is eliminated and the waveform is smoothened by passing the square wave pulse through the LPF.
On the other hand, the power is lost by an amount corresponding to the eliminated high-frequency component, and the frequency deviation point is not reached when the modulation is carried out, as may be seen from FIG. 4A. When the modulated waveform does not reach the frequency deviation point, there is a problem in that the demodulation cannot be carried out normally at the receiver due to the disadvantageous conditions with respect to the receiver.
(2) A description will be given of a case where filters having the same characteristic are used with respect to the frequency deviation of the FSK, by referring to FIG. 5.
FIG. 5 is a diagram showing the frequency deviation for the case where filters having the same characteristic are used. In FIG. 5, the ordinate indicates the frequency and the abscissa indicates the time. When the FSK modulation takes a multiple-value (or multi-level value), the frequency deviation may be made to a plurality of deviation points as shown in FIG. 5. In FIG. 5, L1 through L4 denote deviation points, T denotes a frequency deviation time, t1 denotes a time it takes to reach a demodulation range DM2 having a small frequency deviation, and t2 denotes a time it takes to reach a demodulation range DM1 having a large frequency deviation. In addition, marks "o" indicate sampling points for the case where the frequency deviation is large, and marks "x" indicate sampling points for the case where the frequency deviation is small.
The filter naturally satisfies the characteristic of the leakage power to the adjacent channel. In this case, although t1&lt;t2, the rate of the frequency deviation is constant regardless of the size of the frequency deviation.
On the other hand, it is assumed that the demodulation ranges DM1 and DM2, which are the ranges in which the demodulation can positively be carried out at the receiver, are constant. In this case, the time t1 it takes for the small frequency deviation to reach the demodulation range DM2 is shorter, that is, quicker, than the time t2 it takes for the large frequency deviation to reach the demodulation range DM1, and thus, the demodulation has a sufficient margin such that a highly stable demodulation is possible.
On the contrary, the demodulation becomes difficult on the side of the large frequency deviation, thereby making it the minimum frequency deviation condition. In addition, a quick response is required for the demodulation, but the leakage power to the adjacent channel tends to increase.
FIG. 6 is a diagram showing an example of a frequency deviation condition and characteristic. FIG. 6 shows how a demodulation stability and the leakage power to the adjacent channel are related to the setting of the frequency deviation condition (that is, frequency deviation curve). In FIG. 6, the demodulation stability for the large frequency deviation, the demodulation stability for the small frequency deviation and the leakage power to the adjacent channel are respectively shown for cases where the frequency deviation condition is set so that the deviation point suits the large frequency deviation, the deviation point suits the small frequency deviation, and the deviation point is optimized for the large and small frequency deviations. In FIG. 6, a mark "x" indicates a value outside the range of the standard value, a mark "o" indicates a value within the range of the standard value, and a double circular mark indicates a value which is extremely good and exceeds the standard.
The following can be confirmed by FIG. 6.
First, when the frequency deviation condition suits the large frequency deviation, the demodulation stability is particularly good for the small frequency deviation, and the demodulation stability for the large frequency deviation also falls within the standard. However, the leakage power to the adjacent channels does not satisfy the standard.
Second, when the frequency deviation condition suits the small frequency deviation, the leakage power to the adjacent channel is extremely good and exceeds the standard, and the demodulation stability for the small frequency deviation falls within the standard. However, the demodulation stability for the large frequency deviation does not satisfy the standard.
Third, when the frequency deviation condition is optimized for the large and small frequency deviations, the demodulation stability for the large frequency deviation, the demodulation stability for the small frequency deviation, and the leakage power to the adjacent channel all satisfy the respective standards.
Therefore, the present inventor has found that both the demodulation stability and the leakage power to the adjacent channel can simultaneously satisfy the respective standards by optimizing the frequency deviation condition for each frequency deviation.